1. Field of the Invention
The present invention relates to a communication apparatus having an FFR (fractional frequency reuse) control mechanism using a step function.
2. Description of the Related Art
Recently, high-speed wireless communications have been developed, and the unit price of a frequency spectrum has been raised as the frequency spectrum has been a resource limited to be efficiently used. In the standard of 1xEV-DO, IEEE802.16E, LTE etc., frequency reuse has been a problem to be considered.
FIGS. 1A through 1D are explanatory views showing various aspects of frequency reuse.
The frequency reuse shown in FIG. 1A has a reuse factor of 1 indicating the sectors of all cells having the same frequency F, and high throughput can be attained when all sectors in a cell operate at the same frequency band. However, the reuse method indicates the possibility that a user of a cell edge receives a degraded signal by the interference from an adjacent cell.
To more effectively cover a use of a cell edge, as shown in FIG. 1B, a ⅓ reuse factor (one cell includes three types of frequencies F1, F2, and F3, and each sector uses one frequency) is used, and frequency planning (the arrangement of frequencies is considered during designing) is provided to limit the reuse to a fractional frequency reuse. FIG. 1C shows the case where no frequency planning is used (the arrangement of frequencies is not considered during designing). An intermediate method can be ⅔ frequency reuse as shown in FIG. 1D. The frequency reuse method refers to the use of two of three frequency bands (F12, F23, or F13)) for one sector.
FIG. 2 shows the comparison of performance of each frequency reuse on the basis of the assumption of IEEE802.16e.
In FIG. 2, the horizontal axis indicates longtime average SNR, and the vertical axis indicates the CDF (cumulative distribution function) indicating the cumulative SNR occurrence rate.
As a result, the full frequency reuse (reuse of 1), and ⅓ frequency reuse (reuse of F⅓) with the frequency planning obtain a gain of 10 dB, and the ⅔ frequency reuse (reuse of F⅔) obtains a gain of 3 dB. The gains are obtained by multiplying the gain of the SNR by a frequency use rate, that is, 1 for the reuse of 1, ⅓ for the reuse of F(R)⅓, and ⅔ for the reuse of F⅔.
Aside from the conventional frequency reuse method, some fractional frequency reuse (FFR) methods are proposed as standards.
One proposition as applicable FFR (non-patent document 1) is using a transmission power level with or without restrictions, and a large coverage gain can be obtained by assigning a set of subcarriers without restrictions to a cell edge user. In this method, a predetermined pattern is used, and the predetermined pattern is reported between adjacent cells through a high layer signaling control channel.
Other methods (non-patent documents 2 and 3) are to similarly consider interference control by reserving a subcarrier configured by high layer signaling control channel. The advantage of the interference control is obtained by assigning to a user a preferable frequency depending on the status of the cell closest to the user.
In the exemplified interference FFR scheme, a high layer control channel is normally required to indicate a transmission power pattern or a reserved subcarrier. Therefore, a system without such a control channel is not available.    [Non-patent Document 1] 3GPP TSG-RAN1 WG1 #49, R1-072376, “Further Discussion on Adaptive Fractional Frequency Reuse”, Kobe, Japan, May 7-11, 2007.    [Non-patent Document 2] 3GPP TSG-RAN1 WG1 #49, R1-072411, “Voice over IP resource allocation benefiting from Interference Coordination”, Kobe, Japan, May 7-11, 2007.    [Non-patent Document 3] 3GPP TSG-RAN1 WG1 #50, R1-07-3604, “Semi-Static Interference Coordination Method”, Athens, Greece, Aug. 20-24, 2007.